Without limiting the scope of the invention, its background is described in connection with communication between surface equipment and downhole devices during hydrocarbon production, as an example. It should be noted that the principles of the present invention are applicable not only during production, but throughout the life of a wellbore including, but not limited to, during drilling, logging, testing and completing the wellbore.
Heretofore, in this field, a variety of communication and transmission techniques have been attempted to provide real time communication between surface equipment and downhole devices. The utilization of real time data transmission provides substantial benefits during the production of hydrocarbons from a field. For example, monitoring of downhole conditions allows for an immediate response to potential well problems including production of water or sand.
One technique used to telemeter downhole data to the surface uses the generation and propagation of electromagnetic waves. These waves are produced by inducing an axial current into, for example, the production casing. This current produces the electromagnetic waves that include an electric field and a magnetic field, which are formed at right angles to each other. The axial current impressed on the casing is modulated with data causing the electric and magnetic fields to expand and collapse thereby allowing the data to propagate and be intercepted by a receiving system. The receiving system is typically connected to the ground or sea floor at the surface where the electromagnetic data is picked up and recorded.
As with any communication system, the intensity of the electromagnetic waves is directly related to the distance of transmission. As a result, the greater the distance of transmission, the greater the loss of power and hence the weaker the received signal at the surface. Additionally, downhole electromagnetic telemetry systems must transmit the electromagnetic waves through the earth's strata. In free air, the loss is fairly constant and predictable. When transmitting through the earth's strata, however, the amount of signal received is dependent upon the skin depth (.delta.) of the media through which the electromagnetic waves travel. Skin depth is defined as the distance at which the power from a downhole signal will attenuate by a factor of 8.69 db (approximately 7 times decrease from the initial power input), and is primarily dependent upon the frequency (f) of the transmission and the conductivity (.sigma.) of the media through which the electromagnetic waves are propagating. For example, at a frequency of 10 hz, and a conductance of 1 mho/meter (1 ohm-meter), the skin depth would be 159 meters (522 feet). Therefore, for each 522 feet in a consistent 1 mho/meter media, an 8.69 db loss occurs. Skin depth may be calculated using the following equation. ##EQU1## .pi.=3.1417; f=frequency (hz);
.mu.=permeability (4.pi..times.10.sup.6); and PA1 .sigma.=conductance (mhos/meter).
As should be apparent, the higher the conductance of the transmission media, the lower the frequency must be to achieve the same transmission distance. Likewise, the lower the frequency, the greater the distance of transmission with the same amount of power.
A typical electromagnetic telemetry system that transmits vertically through the earth's strata may successfully propagate through ten (10) skin depths. In the example above, for a skin depth of 522 feet, the total transmission and successful reception depth would only be 5,220 feet. It has been found, however, that when transmitting horizontally through a single or limited number of strata, the vagaries of the strata are small and the media more conductivity consistent which allows for a greater distance of transmission.
Therefore, a need has arisen for a downhole telemetry system that is capable of communicating real time information over a great distance between downhole devices disposed in multiple wellbores using horizontal transmission through a single or limited number of strata. A need has also arisen for such a system that is capable of telemetering the information between the downhole devices and the surface. Further, a need has arisen for a system that uses electromagnetic waves to transmit real time information between downhole devices through a single or limited number of strata and that uses electrical signals to transmit the information between a single downhole device and the surface.